The difference between mean saccade size and scotoma size is a predictive factor for reading speed

Abstract

During the reading with a scotoma, saccade has two functions: progressing and unmasking. The aim of the unmasking is to make visible the masked letters by moving scotoma. So a saccade size at least bigger than scotoma size would allow to unmask all masked letters favours for the word recognition. The study of the difference between scotoma size and saccade size would determine the number of letters still masked after the saccade. We can suppose that kept-hidden letters through a saccade can reduce reading speed. To test this hypothesis, we have evaluated the reading abilities of 12 healthy subjects, with simulated central scotoma using different sizes. We have measured reading speed, saccade size and the mean of number of letters still masked after each saccade. Our results show a mean saccade size that is practically constant, independent of the scotoma size. Thus, we can determine still masked letters after a saccade smaller than scotoma size. The amount of these masked letters has a proportional negative effect on the reading speed: it falls dramatically with an increase of the number of fixation. In conclusion, the impossibility to program efficient eye movements limits the unmasking action penalizing strongly the reading speed.